1. Field of the Invention
The present invention relates to a secondary battery capable of displaying the charge and discharge state thereof.
2. Description of the Prior Art
Recently, various electronic apparatuses of portable type are widely used and various secondary batteries suitable for these apparatuses have been developed and are available in the market.
However, from a practical view point, the prior art apparatuses have problems to be solved. One essential problem is that it is impossible to directly know the charge and discharge state of the secondary battery.
The charge and discharge state can be measured with use of a suitable measuring apparatus, but this method requires a measuring apparatus and is not convenient to each user.
In order to solve this problem, there has been proposed a detection method for detecting the charge and discharge state of the secondary battery which utilizes a color change according thereto (See, for example, JP-A No. 59-16283).
However, in this method, it becomes necessary to provide a display electrode other than essential elements of the secondary battery with use of a material, such as WO.sub.3, TiO.sub.2 or the like, the color of which is changed by a reversible electro-chemical reaction.
Due to this, the battery structure becomes very complicated, and is not advantageous for the production process therefor and the dimension thereof becomes bulky.
On the other hand, the electrochromic device is well known which assumes a specific color due to an electrochemical reversible reaction. Accordingly, the electrochromic device is applicable to a display device.
As an electrochromic display device, there has been known an electrochromic device which utilizes color change due to change in the stage structure of a graphite intercalation compound.
As is well known, graphite has a layered structure capable of forming an intercalation compound between adjacent layers thereof by regularly intercalating various atoms and/or molecules therebetween. The intercalation compound assumes a color specified by the kind of inserted atom or molecule and the stage structure thereof.
In Appl. Phys. Lett. 35 (1978) p 771 and Synth. Met. 3 (1981) p 27, P. Pfluger et al. proposed and tested an electrochromic display device which utilizes a color change caused by a reversible change in the stage structure of lithium-graphite intercalation compound having lithium of an alkali metal as the inserted atomic species.
In this display device, HOPG (highly oriented pyrolytic graphite produced by Union Carbide Corp.) is used for the working electrode, metallic lithium is used for the counter electrode and an organic solvent (liquid electrolyte) such as dimethylsulfoxide or Li.sub.3 N of a solid ion conductor is used for the ion conduction layer which serves as an ion travelling layer for lithium ions.
FIG. 5 shows an example of the electrochromic display device manufactured for trial by P. Pfluger et al. wherein Li.sub.3 N was used for the ion conduction layer.
This device has such a structure that a transparent electrode 32, a working electrode 33, an ion conduction layer 34, a counter electrode 35, an electrode 36 and a glass substrate 37 are formed stacked on a glass substrate 31. When a voltage is applied between two electrodes 32 and 36, lithium ions are implanted from the counter electrode 35 to the working electrode 33 through the ion conduction layer 34 or discharged from the latter electrode to the former electrode to make the color of the working electrode 33 change reversibly. If the electrode 36 is made of a transparent material, the color change of the working electrode can be observed through both of the glass substrates 31 and 37.
However, this type of electrochromic display device using HOPG for the working electrode has some disadvantages as follows.
The device having an ion conduction layer of an organic solvent has an inferior stability even though it has a quick response of about 0.2 second.
HOPG used for the working electrode is apt to take not only lithium atoms but also molecules of the organic solvent used for the ion conduction layer 34 into the layered structure thereof during the action of the device because it has a high crystalline structure and a high orientation. The largeness of each molecule of the organic solvent, when taken into the layered structure, causes the layer distance of HOPG to enlarge considerably and, due to this, the crystalline structure of HOPG is often destroyed. The instability of the display device of this type is caused by the insertion of molecules of the organic solvent into HOPG.
Contrary to the above, in the display device including Li.sub.3 N of a solid ion conductor as the ion conduction layer 34, the response speed thereof is lowered considerably though the problem of the instability is solved.
Further, in this structure of the display device, the color change of the graphite intercalation compound can be observed only from a side opposite to the side being contacted with the ion conduction layer 34 of the graphite working electrode 33. Accordingly, it cannot be observed until lithium atoms inserted from the side being contacted with the ion conduction layer have been diffused in the graphite working electrode 33 to opposite side. This is the reason why the response speed is lowered in this type of the display device.
In order to prevent the lowering of the response speed, it is necessary to make the thickness of the graphite working electrode thinner to an order of several hundred .ANG.. Although it is possible to thin HOPG by exfoliation, it is difficult to obtain a uniform thickness of the graphite thin film of several hundred .ANG. thickness thereby. Further, since this type of display device necessitates complicated formation processes, it is not suitable for industrial production.
Also, the cost of HOPG inevitably becomes very high since it is produced through such a difficult heat treatment to be performed under a high pressure at a temperature above 2,000.degree. C. This makes wide use of this type of display device very difficult since various cheaper display devices are available in the market.